Conventionally, there has been a current control type converter that has a converter control system using current control by the dq axis (refer to, for example, JP3192058).
The current control type converter controls the DC voltage constant by using a q-axis current as an active component, using a p-axis current as a reactive component and controlling the active component by an active current instruction value on the basis of the deviation of the voltage controller for the DC voltage that varies in correspondence with the load. On the other hand, with regard to the reactive component, operation such that a power factor is one is performed by making a reactive current instruction value zero.
In this case, PI controller is used for the active current control system and the reactive current control system, and for suppression of an overcurrent during a startup time, there are the control system shown in FIG. 10 in which the integrator of the q-axis current compensator is initially set, the control system shown in FIG. 11 in which the integrator of the q-axis current compensator and the integrator of the DC voltage compensator are both initially set, and the control system shown in FIG. 12 in which the DC voltage instruction value is initially set smaller than the detected value.
The PI controller 7 of a first current control type converter that has the control system shown in FIG. 10 is constructed of an integrator 18a, a limiter 19a, a proportional multiplier 20a and an adder-subtractor 21a. The PI controller 15a is constructed of an integrator 18b, a limiter 19b, a proportional multiplier 20b and an adder-subtractor 21b. The PI controller 15b is constructed of an integrator 18c, a limiter 19c, a proportional multiplier 20c and an adder-subtractor 21c. In FIG. 10, reference signs 14a, 14b, 17a, 17b denote adder-subtractors, and signs 16a, 16b denote gains.
In the current control type converter, an AC power is turned on in a state in which a PWM signal is stopped, and a smoothing capacitor connected to an output is initially charged up to a diode rectification voltage. Subsequently, the output of the integrator 18b of the PI controller 15a that outputs an active voltage correction value ΔVq is consistently set to a negative value or a negative limit value by PI integrator output initial setting means 23 until a converter startup instruction 22 is outputted. By this operation, the active voltage correction value ΔVq becomes a negative value since the initial value of the integrator 18b is smaller than the output of the proportional multiplier 20b even when an active current instruction value Iq* is positive. Accordingly, an active voltage instruction value Vq* becomes a value close to the maximum value, and a converter input voltage becomes comparatively large even in a diode rectification voltage of which the DC voltage is comparatively low during the converter startup time. As a result, a difference voltage between the power voltage and the converter input voltage becomes small, and no excessive power current flows. When the DC voltage rises and reaches the DC voltage instruction value in the steady state operation, the active voltage correction value ΔVq and the output of the integrator 18b converge almost to zero.
As described above, in the first current control type converter that has the control system shown in FIG. 10, an excessive power current during the converter startup time is suppressed by beginning startup in a state in which the output of the integrator 18b for PI compensation is set to the negative value or the negative limiter before startup of the converter.
Moreover, a second current control type converter that has the control system shown in FIG. 11 differs from FIG. 10 in that the output of the integrator 18a of the PI controller 7 to output the active current instruction value Iq* is set to a negative value or a negative limit value in addition to the setting of the output of the integrator 18b of the PI controller 15a to output the active voltage correction value ΔVq at the negative value or the negative limit value by the PI integrator output initial setting means 23 before startup of the converter. With this arrangement, a summation value of the initial value of the output of the integrator 18a and the output of the proportional multiplier 20a becomes the active current instruction value Iq*, so that the active current instruction value Iq* can be made comparatively small. As a result, a deviation between the active current instruction value Iq* and the active current Iq can be reduced, and the output of the proportional multiplier 20b becomes small. Accordingly, the output of the integrator 18b of the PI controller 15a that outputs the active voltage correction value ΔVq is set to the negative limit value, and therefore, the active voltage instruction value Vq* becomes a value close to the maximum value.
As a result, in the second current control type converter that has the control system shown in FIG. 11, the converter input voltage becomes comparatively large as in FIG. 10, and the excessive power current during the converter startup time is suppressed.
Moreover, a third current control type converter that has the control system shown in FIG. 12 differs from FIG. 10 in that the outputs of the integrators 18a, 18b of the PI controller are set to a negative value or a negative limiter by providing DC voltage instruction setting means 24 and setting a smoothing capacitor DC voltage instruction value Vdc* smaller than a smoothing capacitor voltage detection value Vdc before startup of the converter. Next, converter control is performed by raising the smoothing capacitor DC voltage instruction value Vdc* up to a converter control voltage instruction value together with a converter startup instruction in this state.
In the current control type converter that has the control system of any one of FIGS. 10 through 12 as described above, the integrator is brought into a saturated state by the initial value or the deviation during the startup time, and the output voltage instruction value is lowered with the time constant of the integrator. Therefore, a smooth shift from a rectifier mode to a PWM mode can be achieved.
Since the first through third current control type converters shown in FIGS. 10 through 12 have no steep change in the instruction value, the occurrence of an overcurrent can be suppressed. However, since the PI control is used for the voltage control system and the current control system of different time constants during the startup time, there is a problem that the integrators interfere with each other and an overcurrent occurs due to the overshoot of the current instruction value.
Simulations were conducted in the current control type converters of the constructions shown in FIGS. 10 through 12. In this case, the constants of the main circuit and the control system were set as follows in FIGS. 10 through 12.
Main circuit constants:                Reactor inductance L=3.5 mH;        Reactor resistance r=0.1 Ω;        Smoothing capacitor capacitance C=1000 μF;        DC no-load state        
Power voltages:                Three-phase 400 VAC;        
Control voltage:                700 VDC;        
Voltage control system constants:                Kp=0.1;        Ki=2;        Limiter (±50);        
Current control system constants:                Kp=3.5;        Ki=100;        Limiter (±100)        
In this case, although only the control system that gives the voltage instruction value to the converter is shown, the converter is controlled so that the relation of the following Equation (1) holds on the basis of dq-axis voltages Vd, Vq obtained by the control system.
                                          V            i                    =                                                    V                d                2                            +                              V                q                2                                                    ⁢                                  ⁢                              K            s                    =                                    2                        ⁢                                          V                i                                            V                dc                                                    ⁢                                  ⁢                  ϕ          =                                    tan                              -                1                                      ⁡                          (                                                V                  d                                                  V                  q                                            )                                                          (        1        )            
In this case, since the main circuit of the converter operates as a step-down converter for the DC voltage, the maximum value of a voltage control ratio Ks becomes one, and the circuit operates equivalent to the rectifier mode in the case of the maximum value.
The integral term of the current control system is set negative in FIG. 10, the integral term of the voltage control system is set negative in FIG. 11, and the deviation of the voltage control system is set negative in FIG. 12. Therefore, the active voltage correction value ΔVq is added to a power voltage instruction value Vr*, and the voltage instruction value Vq is set large. Therefore, the voltage control ratio exceeds one according to Equation (1) but limited by the limiter of the maximum value.
FIG. 13 shows a simulation during the startup time conducted for the first current control type converter of the construction of FIG. 10. As shown in FIG. 13, the current control system integrator is initially set at −100 V in this case. Therefore, the power voltage instruction value Vr*=400 V is added, and the initial value of the q-axis voltage instruction value Vq is 500 V. In this case, the current instruction value rises as the PI controller integration value increases since the deviation of the DC voltage control system is positive. Therefore, the deviation of the current control system also becomes positive, and the q-axis voltage instruction value Vq decreases since the PI controller integration value charged negative increases.
Since the converter is in the rectifier mode during the above operation, the DC voltage is charged to about 570 V that is the power crest value. However, since the q-axis voltage instruction value Vq is not lower than 400 V, Ks keeps an upper limit of one as expressed by Equation (1). Therefore, the converter does not perform PWM operation unless the q-axis voltage instruction value Vq falls below 400 V. Therefore, the normal operation such that the control value is changed by the deviation does not function, and the operation is kept to verify the saturated integration value. This state is generally called a reset windup.
A reset windup such that a deviation is accumulated in the integrator of the DC voltage control system also occurs during the above operation. However, since the reset state is removed when Vq falls below 400 V and the converter control is restored, overshoot occurs in the DC voltage while the current instruction value returns to zero.
FIG. 14, which is a simulation result of the second current control type converter shown in FIG. 11, shows both the voltage control system and the current control system initially set. The initial setting value of the voltage control system decreases from the deviation, and the operation after the current instruction value becomes positive is similar to that in the case of FIG. 13 that is the simulation result of the first current control type converter shown in FIG. 10.
Moreover, FIG. 15, which is a simulation result of the third current control type converter shown in FIG. 12, shows the deviation made negative by the voltage instruction value, where an initial value similar to that of FIG. 14 that is the simulation result of the current control type converter shown in FIG. 11 is set by the deviation. FIG. 16 is a simulation result of the current control type converter shown in FIG. 12 where the DC voltage instruction value is gradually increased from the initial value.
As described above, since the rectifier operates as a limiter in the conventional system, a reset windup state is compulsorily established by initially setting the integrator of the current control system, thereby easing a steep change in the instruction value by the integration time constant until removal. However, since errors are similarly accumulated by the reset windup in the voltage control system until the reset windup state of the current control system is removed, the overshoot of the DC voltage occurs at the instance of removal.